This invention relates generally to evaluation of the properties of a substance in a non-destructive, non-invasive, non-contacting manner, and more particularly to evaluation by using magnetic resonance techniques of the property of a substance or a material contained in discrete foil enclosed packages.
Magnetic resonance (MR) has been employed successfully in recent decades in both analytical and clinical sciences involving spectroscopy and imaging applications. MR spectroscopy and less often MR imaging are used as analytical tools in industrial laboratories as frequently as in academic laboratories. The migration of MR from a research laboratory to the manufacturing line and its transformation from a general purpose analytical or imaging technique into a specialized process control subsystem is a much more recent trend.
The generic advantages which have made MR so successful in its traditional role in medicine and research laboratories are equally valuable in manufacturing contexts. These advantages include non-invasive evaluations that are sensitive to many chemical and physical properties of matter, an ability to obtain results without contact and independently, even if the sample container is optically opaque. The requirements and environments of industrial plants as compared to those of hospitals or research laboratories, present new opportunities as well as challenges in using MR technology.
For example, in a manufacturing context there are fewer restrictions on the magnetic fields that can be applied to the sample. For example, the maximum magnetic field strength, the rate of change of magnetic field gradients, and RF heating and acoustic noise emission, can be much higher than allowed in a medical examination. Problems that have existed with noise and claustrophobic effects in MRI imaging of patients at hospitals may not be problems in industrial environments, thus allowing for greater flexibility in MR magnet design.
An unrelenting drive for improved productivity, higher product quality and greater product yield, as well as the wide spread adoption of international manufacturing standards and increasingly stringent governmental regulation, have driven industry to seek novel solutions to automated inspection and control of a diverse assortment of manufacturing processes. Magnetic resonance is becoming more recognized as a solution to many such problems. Many fundamental properties of liquids such as moisture content, bulk flow, diffusion, pH, viscosity, composition and temperature are amenable to analysis through MR techniques. In solids, internal flaws, porosity, uniformity and composition are amenable to analysis. MR has already been explored, for example, in analysis of foods and beverages, plastics and rubbers, petrochemicals, explosives, narcotics, fuel propellants, and even timber. MR is completely non-invasive, non-contacting and does not significantly change the nature of the sample being inspected.
For example, nuclear magnetic resonance (NMR) techniques have been applied as a sorting machine of watermelons. The hardware included an ordinary NMR imaging system (0.5 T, bore size of 68 cm in diameter). Sugar content of a watermelon was detected with high precision, better than 0.07 percent, by applying multiple regression analysis to observed relaxation times T1 and T2. Internal defects such as voids in watermelons have been detected by analyzing NMR signal intensities in an image. Voids can be detected and sorted with high accuracy and high speed i.e., at a rate of inspection of one second per piece.
The underlying concepts of magnetic resonance spectroscopy and magnetic resonance imaging are now generally well known and are not presented in detail herein. The basic physical concepts of MR and specialized techniques for three-dimensional imaging are described in NMR Imaging In Medicine by Ian L. Pykett (an inventor here), an article published in Scientific American, Volume 246, No. 5, May 1982, pages 78-88.
The published article Application Of Magnetic Resonance In Food Science, by Schmidt, Sun and Litchfield, Critical Reviews In Food Science And Nutrition, 36(4):357-385 (1996), also reviews the physical principles of MRI, describes various applications using MRI imaging, and illustrates the rapid progress in the art during the short interval between publication of the two above-referenced articles.
Current Methods: Spoilage Detection
The patent of Schenz et al., No. 5,270,650, issued Dec. 14, 1993, discloses techniques for non-destructive detection of spoilage in foodstuffs using nuclear magnetic resonance spectroscopy. The '650 patent is concerned with detection and measurement of bacterial activity within foodstuff containers. Bacterial action and foodstuff spoilage occurs, although not frequently, even in packaged foodstuff that has been sterilized after initial packaging or has been packaged under aseptic conditions. Prior to the spectroscopy methods described in the patent, quality control of packaged food stuff was maintained by several methods including a destructive method requiring the opening of a randomly selected container in order to test the nutritional product contained therein. The pH level is often considered an indicator of bacterial action, a drop in pH often indicating that the product is not sterile. This sampling is a destructive and time consuming test, that in high speed production, is only applicable to a limited number of containers out of a large batch of product.
Visual inspection for sterility may be possible when the product is in a container having transparent or translucent walls. This method is labor intensive and frequently inconclusive since the inspector may not always be able to detect visually the difference between contaminated and non-contaminated product.
Subculture methods from a small batch sample are unreliable and extremely late in providing results relative to a large production run, which is done at high rates of output.
There are many other tests for sterility described in the '650 patent; these tests can be replaced by non-invasive, non-destructive, rapid MR spectroscopy. In the patent, a nutritional product in a sealed container, transparent to RF, is inspected by MR techniques that determine the peak free induction decay value associated with the nutritional product. Changes in that property with passage of time, when that property is measured under similar conditions, indicate whether spoilage of the nutritional product has occurred. Alternatively, the determination of spoilage may be based on a comparison of immediate test results with previously established standards for that product.
The two articles and U.S. patent mentioned above are incorporated herein by reference.
MR inspection techniques cannot be applied in every situation. For example, MR was not considered to be applicable in the prior art to evaluation of samples which reside inside metallic or ferrous containers. Such containers generate interactions with the static magnetic field of the MR system and deleteriously affect the MR response signal. MR has also not been applied to some systems which contain nonferrous but electrically conducting materials, as such materials may severely attenuate and affect the homogeneity of the radio frequency magnetic field, inhibit field penetration into the sample, or generate undesirable eddy currents during a switched gradient operation, resulting in degradation of the resulting data.
Thus, it has not been considered possible to do MR inspection of a product that is entirely enclosed in a metal enclosure or even an enclosure including a complete barrier of metallic foil. It is well known in the prior art that an enclosure of metal, or of any electrically conductive material, provides a shield for the enclosure's contents from external electrostatic and electromagnetic fields. For example, pending application Ser. No. 08/974,291, filed Nov. 19, 1997, having several joint inventors who are inventors in the present application, describes an inspection system using MR pulse sequences for simultaneous inspection of multiple packages of food product with the packages positioned in a three dimensional physical configuration of the final shipping carton. Each package is individually, non-invasively evaluated without contact and already within the final shipping carton. Indication of any deviant package is provided.
However, the pending '291 application, which is hereby incorporated by reference, makes special note that only product packages that are at least partially transparent to RF magnetic fields operate successfully in the apparatus. Use of the apparatus to inspect product that was entirely or substantially enclosed in a metal or foil container was not considered possible at the time of the invention in the pending application. Thus, it was considered that many food products, which are preferably packaged in containers made of or including metallic foil as a barrier to atmospheric leakage, were never considered as subject to inspection with the MR apparatus of the pending application.
In summary, the "wishful thinking" of perhaps only 30 years ago for non-invasive, non-destructive testing of many materials and processes including agricultural products and packaged foodstuffs, has become realizable. Improved productivity, higher product quality, and greater product yield are now available by means of assessments at high speeds, substantially in real-time, and with a high level of reliability. Unfortunately, available MR inspection equipment and its inability to measure quality of product in metal cans and metallic foil enclosures, has limited use of such MR equipment and denied manufacturers, for example, in food industries, of the tremendous advantage of these non-invasive, high speed techniques.
What are needed are quality evaluation methods and associated apparatus for accelerated MR inspection for single or multiple characteristics of finished product, which methods and apparatus are non-destructive, non-contacting, and non-invasive, and operate on products in containers having an electrically conductive barrier separating the product from the ambient environment, a substantial portion of the barrier being positioned transversely to the RF magnetic field orientation during MR excitation.